Interpretive Summary: The pollen and nectar that female solitary bees collect from flowers is used to feed both themselves and their progeny. Adults primarily consume nectar but feed both nectar and pollen to immatures. Solitary bee females must be especially efficient at foraging because they are solely responsible for collecting these foodstuffs (there are no workers to help). .Foraging efficiency involves integrating flower density, amount of pollen and nectar supplied per flower, and distance of flower patches from the nest. We studied the solitary blue orchard bee, an important pollinator of orchard crops, to try to understand its foraging behavior. We selected for study two sites which contained two preferred flower species of the blue orchard bee: willows and waterleaf. Nests with actively foraging bees were positioned so as to be at different distances from the willow and waterleaf populations so we could assess the effect of distance and density on foraging. Contrary to expectations, bees consistently mixed the pollen of the two plant species on their collecting trips, i.e., instead of focussing on the species that was closest and therefore most efficient to forage from they commonly visited both species. We interpret this finding to be the result of a mixed foraging strategy necessitated by the complementary resource production of the two plant species: willow supplies more pollen but waterleaf produces more nectar. In the interests of efficiency, bees collect waterleaf pollen when they visit those floweres for nectar. Crop species that the blue orchard bee is invited to pollinate must supply adequate amounts of both pollen and nectar to optimize visitation by this bee.

Technical Abstract:
When collecting food, females of many solitary bees must often balance alternative demands such as energy intake and nutrient requirements. Female bees are usually confronted with a choice among several flower species that differ in their location and abundance within the community, and in the efficiency with which their pollen and nectar can be harvested. By manipulating nest locations of Osmia lignaria in natural populations, we investigated the effects of distance and flower density of two flower species on pollen collection. Distance and density both contribute to the profitability of a species. Distance predictably affected pollen use. Bees collected significantly more pollen from the plant species that grew nearest to their nests. Flower density had a less predictable impact on pollen use. Foraging choices were not determined by energetics or pollen resources alone. Bees consistently mixed pollen from more distant species despite substantial added foraging costs, and also mixed when one species was an order of magnitude rarer. Because bees require nectar as well as pollen to feed their offspring, nectar rewards can potentially affect pollen-collecting choices. In this system, the efficiencies of harvesting nectar and pollen rewards are inverted between flower species, which favors visitation to both. Bees appear to collect pollen from the low-pollen, high-nectar plant while visiting it for nectar. Thus, a nectar-collecting constraint may favor collecting pollen from mixtures of species.